The invention relates to a total knee-joint prosthesis comprising a tibial part, a femoral part and a tibial insert for joining the tibial part to the femoral part. The invention also relates to a tibial insert of this kind.
A similar total knee-joint prosthesis is already known from European Patent Application No: 92 113726.1 in the name of AESCULAP AG. According to this prior-art document, the total knee prosthesis is made up of a tibial part comprising a tibia plate fixed to the proximal part of the tibia by an anchoring rod anchored in the medullary cavity cc the tibia, a femoral part comprising two condyles fixed to the distal part of the femur, the condyles having spherical outer surfaces, and a tibial insert disposed on the tibial plate and on its upper part having surfaces complementary in shape with the surfaces of the condyles so that the condyles can slide relative to the tibial insert and so that the tibial part can bend relative to the femoral part. A means is provided for coupling the tibial insert to the tibial plate and comprises a journal mounted pivotably in a recess formed at the centre of the tibia plate, and an appendix which cooperates with a groove formed in the bottom part of the tibial insert so as to guide the tibial insert in translation and in rotation in the plane of the tibial plate relative to the said tibial plate.
This total knee prosthesis has the disadvantage that when the prosthesis is fitted, the practitioner must centre it perfectly in position so as exactly to correspond to the anatomy of the femur and the tibia. The slightest error in centring will make it necessary for the practitioner to start again.
The invention relates to a total knee prosthesis comprising a tibial insert between a tibial plate provided on its inner surface with means for anchoring to a proximal end of a tibia, and a femoral plate comprising condyles provided with means for anchoring to a distal part of a femur, the tibial insert being slidably mounted on the tibial plate and comprising upper surfaces complementary in shape with outer surfaces of the condyles,
a tenon being mounted so as to project from the outer surface of the tibial plate and comprising a base and an oblong part extending from the base parallel to the outer surface of the tibial plate; and
the tibial insert comprising a cavity in which the oblong part engages with possibility at least of rotation of the oblong part relative to the tibial part, characterised in that the size and shape of the cavity are such that the base of the tenon in the cavity can move relative to the tibial insert in antero-posterior and/or medio-lateral translation.
Thus, the prosthesis is centred dynamically by simple medio-lateral movement of the insert relative to the tibial plate when the prosthesis is fitted.
The prior-art total knee prosthesis have the further disadvantage that when the knee is bent, owing to uncontrolled movements, the journal may come loose from the bore in which it is mounted in the tibial plate, resulting in undesirable separation of the tibial insert and the femoral part from the tibial plate.
Also, rotation of the tibial part relative to the tibial plate is not limited, since the coupling means can make a complete rotation around the bore, which does not correspond to the rotation normally observed in the case of the anatomical knee.
According to an improvement of the invention, the tenon is fixed to the tibial plate.
Another reason for fixing the tenon to the tibial plate is to prevent the tenon coming loose from the bore formed in the tibial plate. In order however to allow the tibial insert to rotate relative to the tibial plate, a cavity is provided sufficient in size to permit rotation of the oblong part of the tenon in the cavity over a given range of rotation, e.g. xc2x125xc2x0 or preferably xc2x115xc2x0.
In a preferred embodiment, the oblong part is at a distance on the tibial plate, the cavity comprises a first lateral opening formed in the anterior lateral wall of the tibial insert and a second rear opening, the lateral opening being of a size allowing the oblong part to move through it and the rear opening being bounded by bottom edges of the lateral walls of the cavity and by a bottom edge of the bottom rim defined by the lateral first opening of the abutment, so as to form a hole which limits the sliding motion in antero-posterior or medio-lateral translation of the base of the tenon relative to the tibial plate, and the bottom rim of the first opening forms an abutment which cooperates with the oblong part so as to prevent upward detachment of the tibial insert from the tibial plate.
The abutment is provided so as to prevent anterior luxation of the insert relative to the tibia if the posterior crossed ligament (PCL) is cut, since the abutment takes over the function of the PCL of limiting anterior movement when the knee bends.
The invention also relates to a tibial insert as used in the tibial knee prosthesis described hereinbefore.